Enzymatic carboxyl methylation of proteins is a post translational modification. Carboxyl methylation reduces negative charges on proteins and alters their structure and properties, providing an important regulatory mechanism. Protein carboxyl methylation is associated with exocytotic release, the latter dependent on depolarization of the cell membrane and Ca++ movements. Posterior pituitary hormones vasopressin and oxytocin and their corresponding neurophysins are released by exocytosis which occurs upon depolarization. The homozygous Brattleboro rat lacks vasopressin and its associated neurophysin, and presents the syndrome of diabetes insipidus. Both homogenates of posterior pituitary and whole posterior lobes incubated in vitro show a large decrease in endogenous methyl acceptor proteins in homozygous Brattleboro rats. Physiological and biochemical studies demonstrated that most of the decrease in endogenous methyl acceptor proteins in these animals in the result of the absence of vasopressin-associated neurophysin. Protein carboxyl methylase activity is high in homozygous Brattleboro rats, a change probably correlated with hyperactivity of the neurosecretory posterior pituitary system. These data indicate a close association between protein carboxyl methylation and posterior pituitary neurosecretion. In addition, other, still unidentified proteins occur as endogenous methyl acceptor proteins in the rat posterior pituitary.